1. Field of the Invention
This invention pertains to a transmission amplifier for radio communications which adaptively changes the frequency band. In particular, it pertains to a band selection type feed forward amplifier for multiple frequency bands which selects and amplifies an arbitrary frequency band from among a plurality of frequency bands.
2. Description of Related Art
The base configuration of a feed forward amplifier is shown in FIG. 1. The feed forward amplifier includes two signal processing circuits. One is a distortion detection circuit 100 and the other is a distortion elimination circuit 101. Distortion detection circuit 100 is composed of a main amplifier signal path 103 and a linear signal path 104. Distortion elimination circuit 101 is composed of a main amplifier output signal path 108 and a distortion injection path 109. Main amplifier signal path 103 (also called a vector adjustment path) is composed of a vector adjuster 105 and a main amplifier 106. Vector adjuster 105 has a variable phase shifter 105a and a variable attenuator 105b. Linear signal path 104 is composed of delay lines. Also, main amplifier output signal path 108 is composed of delay lines. Distortion injection path 109 is composed of a vector adjuster 110 and an auxiliary amplifier 111. Vector adjuster 110 is composed of a variable phase shifter 110a and a variable attenuator 110b. A divider 102, a power combiner/divider 107, and a power combiner 112 are simple lossless power dividers and power combiners composed of transformer circuits, hybrid circuits, and the like.
First, an explanation of the basic operation of the feed forward amplifier will be given. The signal input into the feed forward amplifier is divided into main amplifier signal path 103 and linear signal path 104 by means of divider 102. At this point, variable phase shifter 105a and variable attenuator 105b of main amplifier signal path 103 are adjusted so that the signals of main amplifier signal path 103 and linear signal path 104 have equal amplitude and opposite phase. As methods for bringing the paths to opposite phases, there is the method wherein power divider 102 or power combiner/divider 107 sets a phase shift quantity appropriately between the input and output terminals or the method wherein main amplifier 106 inverts the phase.
Since distortion detection circuit 100 is configured in this way, power combiner/divider 107 can output the differential component of the signal passing through main amplifier signal path 103 and the signal passing through linear signal path 104. This differential component is precisely the distortion component generated in main amplifier 106. Due to this fact, the block from power divider 102 to power combiner/divider 107 shown in FIG. 1 is called a distortion detection circuit.
Next, an explanation regarding distortion elimination circuit 101 will be given. The output of distortion elimination circuit 100 is divided, via power combiner/divider 107, into main amplifier output signal path 108 and distortion injection path 109. The output of main amplifier 106 from main amplifier signal path 103 (the signal passing through main amplifier signal path 103) is input into main amplifier output signal path 108. Also, the distortion component of main amplifier 106 detected in distortion detection circuit 100 (the differential component of the signal passing through main amplifier signal path 103 and the signal passing through linear signal path 104) is input into distortion injection path 109. As for variable phase shifter 110a and variable attenuator 110b of distortion injection path 109, the distortion component of the signal passing through main amplifier output signal path 108 and the signal passing through distortion injection path 109 are adjusted so as have equal amplitude and opposite phase. By making an adjustment in this way, power combiner 112 can combine the signal passing through main amplifier signal path 103 with the distortion component of main amplifier 106 having equal amplitude and opposite phase. And then, power combiner 112 outputs a signal in which the distortion components of the whole amplifying circuit are cancelled. Further, even if it is a matter of common knowledge, a linear amplifier is used as an auxiliary amplifier in order to eliminate the distortion component generated in the main amplifier used in a feed forward amplifier. The aforementioned operation is an ideal operation of a feed forward amplifier. In practice, it is not simple to completely maintain a balance of the distortion detection circuit and the distortion elimination circuit. Also, even if tentatively the initial settings are perfect, since the properties of the amplifier change due to fluctuations in ambient temperature, power supply, and the like, it is extremely difficult to preserve an excellent balance which is stable over time.
As a method for maintaining with high accuracy a balance of the distortion detection circuit and the distortion elimination circuit of this feed forward amplifier, there is known an self-adjusting method using a pilot signal. E.g., there exist the Japanese Patent Application Laid-Open Publication No. 1 (1989)-198809 (Patent Reference 1) and the like. As devices putting these methods into practical use, there is known the article “Extremely Low-Distortion Multi-Carrier Amplifier For Mobile Communication Systems-Self-adjusting Feed-Forward Amplifier (SAFF-A)” by Toshio Nojima and Shoichi Narahashi, Institute of Electronics, Information, and Communication Engineers, Wireless Communication Systems Society, RCS90-4, 1990 (Non-patent Reference 1). These feed forward amplifiers were put into practice in the 800 MHz band and the 1.5 GHz band of the PDC (Personal Digital Cellular) mobile communications standard in Japan. This kind of feed forward amplifier is generally designed and adjusted to amplify separately for each frequency band.
In the radio systems developed this far, a single system in accordance with any one of PDC, GSM (Global System for Mobile communications), IMT-2000 (International Mobile Telecommunications 2000), and the like, was used. As against this, there is the technology of carrying out a transfer to software of some functionality of radio devices so that it becomes possible for a single hardware to handle a plurality of radio systems. If it is possible for a single hardware to handle a plurality of radio systems, the user can use the mobile communication environment without any awareness of the radio system or the core network in the background thereof. However, a single hardware actually handling a plurality of radio systems is something that has not reached implementation.
Also, it can be considered that, for each region or operator, the services offered with the radio system will be different and that the radio systems will also gradually become diversified. For this reason, it can be considered that, in the future, there will arise a need to make radio systems coexist which are optimal for each purpose, at the same time and in the same place.
As a method of using these plural radio systems, there is the multiband radio system. This radio system adaptively changes the frequency band used or the number of frequency bands used in response to the propagation environment and the traffic conditions. Also, in order to ensure a prescribed transmission quality or transmission volume, multiband transmission using frequency bands not in use is effective. Consequently, in a multiband radio system, in order to ensure the transmission quality or transmission volume to be guaranteed by the same radio system, the number of frequency bands is changed. Moreover, changes are also carried out in the same way within the same frequency band. Further, a multiband radio system, in case there coexist frequency bands used by several operators, can raise the frequency utilization efficiency by carrying out adaptive control using available frequency bands by means of interference recognition technology, frequency sharing technology, interference cancellation technology, produced interference reduction and avoidance technology, multiband control technology, and the like.
The feed forward amplifier is used as a linear amplifier for base stations handling multiband radio systems like this. However, in case the plural frequency bands to be amplified are widely separated, compared to the bandwidth of each frequency band, the adjustment levels of the variable phase shifter and the variable attenuator for keeping the balance of the distortion detection circuit and the distortion elimination circuit within a prescribed range vary with the frequency band to be amplified, because the electrical length of the delay line for each frequency band differs.
To put it in concrete terms, in case a delay line is used in common for all frequency bands, there is, due to the frequency differences of the input signals, ordinarily a need for the setting value of the vector adjuster to track a signal rotating with the angular velocity of the frequency difference. However, in the vector adjusters developed this far, it has not been possible to track a signal rotating at a velocity like that. Also, as for the vector adjusters discussed this far, it has not been possible to simultaneously set an optimal amplitude and phase, with respect to plural input signals, for structural reasons.
E.g., in case 800 MHz band and 1.5 GHz band signals are input into the same vector adjuster, it is possible to carry out optimal vector adjustment with respect to any one of the frequency bands. However, it is not possible to carry out optimal vector adjustment which tracks a frequency difference of 700 MHz. Consequently, the conventional feed forward amplifier has not been able to simultaneously amplify the 800 MHz band signal and the 1.5 GHz band signal at or below a prescribed distortion compensation level.
As a method of resolving this, a dual-band feed forward amplifier is proposed in the article “Dual-band Feed Forward Amplifier” by Yasunori Suzuki and Shoichi Narahashi, the 2005 General Meeting of the Institute of Electronics Information and Communication Engineers, C-2-2, March 2005 (Non-patent Reference 2). With this configuration, there is provided a vector adjuster having a band extraction means for each frequency band. In other words, this dual-band feed forward amplifier extracts the signal of the vector adjusted frequency band from the signals of two frequency bands input by means of a filter provided in a pre-stage of the vector adjuster. And then, vector adjustment is carried out for each frequency band. This dual-band feed forward amplifier configuration is capable of distortion compensation in a plurality of frequency bands. Further, the compensated band is fixed by the filter.
In multiband radio systems having a plurality of transmission bands, it can be considered to change the frequency band due to the service situation of the radio system, interference of other radio systems, and the like. However, as mentioned above, the bandwidth of the distortion compensation of the feed forward amplifier is determined by the adjustment accuracy of each loop of the distortion detection circuit and the distortion elimination circuit. Consequently, in the conventional feed forward amplifier, the adjustment of distortion compensation could not be made to correspond with the frequency band changes. Nor was it possible for the conventional dual-band feed forward amplifier in which the distortion compensated frequency band was fixed to adaptively change the operating frequency. For a feed forward amplifier used over a long time, the change in frequency band accompanies repairs or a change of the feed forward amplifier in the base station. Consequently, an enormous amount of labor and time is required to readjust a large number of feed forward amplifiers. A feed forward amplifier configuration making this kind of labor and time expense unnecessary was required.
E.g., in case, for a dual-band feed forward amplifier which simultaneously compensated the distortion of a signal in a frequency band f1 and a signal in a frequency band f2, the frequency band was changed from f2 to f3, it was not possible to simultaneously compensate the distortion of the signal in frequency band f1 and the signal in frequency band f3. This was so because loop adjustment by the frequency difference of f1 and f3 was not possible, as mentioned above, due to the fact that the operating frequencies of a conventional dual-band feed forward amplifier are fixed.
Also, there can be considered the method of providing, in the dual-band feed forward amplifier, fixed filters and vector adjusters handling all the frequency bands that may be thought to be available for future service. However, having fixed filters and vector adjusters able to handle all the frequency bands amounts to having fixed filters and vector adjusters which are not used something which runs counter to configuring an economical feed forward amplifier. There was demanded a feed forward amplifier with no need for the exchange of constituent parts and having no redundancy to accompany in this way the changes in frequency band or the increase and/or decrease in the number of carrier waves.